Thermionic cathodes



United States Patent 3,013,171 THERMIONIC CATHODES Arnold Hugh William lieclr, London, England, assignor to International Standard Electric Corporation, New

York, N.Y., a corporation of Delaware Filed May 3, 1956, Ser. No. 582,595

Claims priority, application Great Britain June 23, 1955 12 Claims. (Cl. 31382) The present invention relates to thermonic cathodes whose emitting area is required to be very small, the cathode being of the dispenser type disclosed in.U.S. application Serial No. 446,206, filed July 28, 1954, now Patent No. 2,912,611. Cathodes of the kind there disclosed are formed from a compressed and sintered mass of refractory metal powder, for example, nickel, mixed with thermionically emissive material such as, initially, standard double or triple alkaline earth carbonates. These cathodes can be machined either before, or preferably after activation in a simple diode, only a short reactivation in the completed valve being required after such turning or grinding treatment.

At millimetre Wavelengths valves are required having cathodes with very small emitting areas, for example 3.0 mm. x 0.4 mm. in a typical application, together with very high current densities. Such small cathodes are not readily obtained with normal techniques.

It is an object of the invention to provide a thermonic cathode of very small dimensions and capable of yielding high current densities.

According to one aspect of the present invention there is provided a thermionic cathode of the kind in which electrons are emitted from active material which is a compressed and sintered mass of refractory metal powder mixed with thermionically emissive material and in which the desired emitting surface is of very small area, characterised in this that the said emitting surface occupies the end of a block of said active material which projects from the face of a larger block of sintered metallic powder.

According to another aspect of the invention there is provided a thermionic cathode comprising a metal tube adapted to house a cathode heater, a block of compacted and sintered metallic material held in the said tube, a 1 rojection from the face of the said block formed of active material which is a compressed and sintered mass of refractory metal powder mixed with thermionically active material, and an outer metal shield enclosing the said cathode except for a clearance aperture about the end of the said projection.

Embodiments of the invention will be described with reference to the accompanying diagrammatic drawings in which:

FIG. 1 shows a longitudinal section through a cathode according to the invention;

FIGS. 2 and} are alternative plan views of the cathode of FIG. 1; 7

FIGS. 4 and 5 show" possible configurations for the emitting surface of acathode of the invention and the l a heat shield for the body of the cathode.

from the main body. of the cathode.

In all the figures of the drawings, the details of the cathode heater, with which we are not immediately concerned, have been omitted.

Referring now to FIG. 1, a tube 1 adapted to house a cathode heater, and made of nickel or one of the proprietory refractory alloys having a high nickel content, serves as a holder for a block 2 of compacted and sintered nickel powder mixed with barium and strontium carbonates together with a small amount of Zirconium hydride to act as a reducing agent. A projection 3 is formed on the face of the block, by grinding in the case of a circular cathode as in the plan view of FIG. 2, or by milling if a rectangular shape is required, such as in FIG. 3. The processing and activation of the cathodes is the same as is disclosed in the specificationof the aforementioned application Serial No. 446,206, the machining operation being performed before or after preliminary activation, as desired.

To complete the cathode an outer metal shield 4 encloses the 'body of the cathode but for a central aperture 5 about the projection 3. The shield 4 in the embodi ment of FIGS. 1 to 3, slips over the body of the cathode and is welded thereto. The eflective emitting surface of the cathode, namely the face of the projection 3, may be in front of the shield, in the same plane as, or behind the shield, as dictated by electron optical requirements.

In FIGS. 4 and 5 the shield 4 is provided with a concave face about the projection 3, and in FIG. 5 the face of the projection 3 is also concave, these parts being shaped to form part of a focussing electron optical system.

In FIG. 6 the shield 4 is shown as insulated from the body of the cathode by means of washers oand 7 located by means of flanges 8 and 9 secured to the tube 1. If desired the shield 4 and the projection 3 may obviously be shaped as in FIG. 4 or 5. V

In FIG. 7 a modification of the body of the cathode is shown in which the tube 1 is provided with a flange 10 which forms seatings, on the one side for the block 2, and on the other, if required, for an insulating washer 7 to space the body from its surrounding shield.

In the embodiments so far illustrated, the block 2 is made of active material, the projection 3, however, being the essential electron emitting portion of the cathode. The excess of active material may be disadvantageous, not only from the point of view of economy, but also by permitting an excessive amount of barium to be evaporated into the valve during use. An alternative construction is illustrated in FIG. 8 in which the block 2 is formed of sintered nickel, without admixture with emissive material, and, in place of the projection 3 of the previous embodiments, a small plug 11 of active material is let into the block 2 to provide the projecting emitting surface. powder mixture for material of the plug 11 can be introduced into the recess with the aid of a" superimposed collar, the plug 11 being compressed and sintered sep arately. Alternatively the two powders can be placed 'ina suitable die so that the block 2 and plug 11 are pressed and sintered as a single unit.

A further modification may be made to the construction, if desired, so that the cathode shield 4 also acts as In the embodiment of FIG. 9, the tube 9 is enlarged in diameter at its lower end so as to enable the shield 4 to be spaced In this case the shield should be thin-and formed of an alloy of low thermal conductivity.

.In FIG. '10 is shown, diagrammatically, an electron gun'employing a very large ratio of cathode to anode radius, which is necessary in very fine focus guns. The cathode structure is similar to that shown in FIG. 1 except Patented Dec. 12, 1961 The block 2 may first be formed and thenthe that an additional concave focussing electrode 12 is Welded onto the front of the shield 4. The anode comprises a disc 13 having a rearward tubular projection 14 forming an elongated passage for the electron beam. The entry to this passage is approximately at the centre of curvature of the focussing element 12 and its length is such as to provide a noise reducing drift tube at the midband of the desired frequency of operation of the valve for which the gun is designed.

In embodiments of the present invention, since the overall size of the cathode, including the shield, is not especially small, conventional means of centering and securing it in position are adequate, in spite of the fact that the effective emission surface is of such small dimensions that very great difficulties would be encountered in manufacturing and assembling a cathode of previously known type.

A very important technical feature of cathodes of the invention is that the emitting surface is completely defined. It is impossible for the emitting area to increase, as would be the case for instance, if the shield were in contact with the emitting surface. At the same time the difficulties of supporting very small strips or plugs of active cathode material have been overcome.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

I claim:

1. A thermionic cathode comprising a compressed and sintered relatively large block of refractory metal powder mixed with thermionically emissive material and a smaller block of sintered thermionic emissive material having very small area integral with and projecting from the end of said relatively large block to form the active surface of said cathode.

2. A thermionic cathode assembly comprising a metal tube adapted to house a cathode heater, a block of compacted and sintered metallic material held in the said tube, a projection from the face of the said block formed of active material which is a compressed and sintered mass of refractory metal powder mixed with thermionically active material to form the active emitting cathode surface, and an outer metal shield enclosing the said block and projection except for a clearance aperture about the end of the said projection.

3. A thermionic cathode assembly according to claim 2 in which the end of the said projection is concave.

4. A thermionic cathode assembly according to claim 3 in which the said shield has a concave face about the said projection.

5. A thermionic cathode assembly according to claim 2 in which the said shield is electrically insulated from the body of said block and projection.

6. A thermionic cathode assembly according to claim 2 in which the said shield is adapted to form a heat shield for the body of the said block, the said metal tube having an enlarged portion to space the shield away from the body of the cathode.

7. A thermionic cathode assembly according to claim 2 in which the said metal tube comprises an end flange forming a seating for the said block.

8. A thermionic cathode assembly according to claim 2 in which an apertured focussing electrode is secured to the end of the said shield to project as a concave surface therefrom, the said emitting surface projecting into the aperture of the said focussing electrode.

9. A thermionic cathode assembly according to claim 2 in which the said block and the projection therefrom are of the same material and integral with one another.

10. An electron gun comprising a cathode assembly according to claim 8 and an anode in the form of a disc having a central tubular projection directed towards the cathode to provide a narrow passage for the electron beam.

11. A vacuum tube comprising a sintered cathode block having an electron emitting projection thereon, a curved focussing shield adjacent said projection, and an anode having a central tubular projection extending towards said cathode to provide a narrow passage for the electron beam.

12. The vacuum tube of claim ll, wherein the end of said tubular projection is located at the center of curvature of said focussing shield.

Cook June 2, 1953 Cook June 2, 1953 

